Some optical devices may include a waveguide that is intended to be coupled to another waveguide or fiber having a significantly larger cross-sectional size. For example, a planar lightwave circuit (PLC) can have a waveguide on the order of four microns in width to be coupled an optical fiber with a diameter of about ten microns. One way to couple a port of a relatively large waveguide to a port of a significantly smaller waveguide is by forming a tapered waveguide structure to couple the two waveguides. In one type of taper, the taper at one end has a width or diameter of about the same size as the larger waveguide. At the other end, the taper comes to a point. The sides of the taper are typically straight so that the taper has a wedge-like shape, with the taper narrowing from the wide end to the point or narrow end. The wide end of the taper is used to couple the taper from the larger waveguide. The idea behind this taper is to create a virtual, vertical effective index change in the waveguide that forces the mode into an underlying, single-mode waveguide. As the taper becomes narrower, the effective index decreases, and the mode moves lower in the semiconductor material.
One conventional technique to form the above-described taper when the smaller waveguide is a semiconductor waveguide is to etch one end of the smaller waveguide to form the taper. For example, at the end of the waveguide, the smaller waveguide has: (a) a length about equal to the desired length of the taper; and (b) a thickness that is about equal to the sum of the desired thickness of the smaller waveguide and the desired thickness of the taper. For example, the resulting thickness can be about the height of the core of an optical fiber. This end of the smaller waveguide is then etched using standard etching techniques to form the taper with a shape as described above. However, some etching processes form the taper's point so that it appears eroded, instead of the desired sharp edge or point. This erosion can degrade performance of the taper. In addition, typical etching processes cause the etched surfaces to be significantly less smooth than the surfaces that are not etched. This roughness can increase the waveguide's loss (e.g., in some tests the etched surfaces increased loss an addition five to ten decibels).